Support multi-page layout

[dalogsdon]

https://github.com/jupyter-incubator/dashboards/issues/206 will add a multi-page dashboard layout:

Multi-page mode: a higher-level abstraction where all visible cells are shown one after the other (in notebook order), one at a time. This view takes care of setting up the "pages" of the dashboard and provides the functionality to go next/back, including stylable UI (buttons, progress bars, etc).

We should support this layout once it is implemented in the dashboards notebook extension.